Let me explain, the Scp would consist of a coin (Or collections of coins) made each one of different metals found in South America like copper, tin, zinc. etc. that in presence of gold would changing the gold at an atomic level, taking it all its value.
The collection was going to be send to a Spanish nobleman of the beginnings of the 16th century, a man very tied to the Catholic Kings, it was a vengeance made by a old native of the New World for the Spanish conquest. This is the rough idea, maybe I would change details.
Of course, ths would be created as a simple story, I have no intent of making a polemic article or nohing like the style, I just found the period interesting, nothing else.
Edit: I would write it as a virus that targets gold, more or less.

Scientifically speaking, it would be difficult to pull off since you are making changes at an atomic or possibly sub-atomic level that would involve a MASSIVE amount of energy. You would either have to pump a huge amount of energy into it, or it would release an equal amount of energy as part of the reaction. I don't know if it would be feasible to do. Now, a virus or bacterium that utilizes gold for food… That would be possible.

Maybe I'm looking at this too much from a defeatist angle, but the huge problem with this is that the perceived value of gold and any other given metal in the old day was more or less what the authorities wanted it to be. While its low reactivity to rust and its high density were indeed factors that made it the most sought metal, it was not until relatively recently that those had that many practical uses beyond jewelry and non-decay.

Basically speaking, in politics, if some super power like Spain suddenly found itself with tons of fake gold, they would make sure it was "valuable" by military force.

It not creates gold, it destroy it, it takes the gold and it converts it in zinc, tin and copper "eating" the gold, and making the result his medium of transport. The problem is that it never reached its destiny and now the foundation have found a virus that can reproduce eating precious metals and that could wreck the economy.

Yeah, I got that. What I meant is that in terms of economics and feudal politics, a shortage of gold in Spain would have merely meant either them stealing more gold or forcing others to take the other metals as a form of payment. Unlike stuff like oil (or for a matter of fact, gold in our current era), there wass little "instrinsic" value to precious metals.

Made me think of some story where the protagonists are trying to stop the villain from getting his hands on a devastating magic spell and failing- only to discover that the spell in question was a mass famine spell which was potentially world-ending in the days of subsistence farming but more or less useless in the modern day.

Maybe you could play this from that angle? A microbe or curse intended to destroy a nation's currency designed by someone who clearly didn't understand very well how economics worked.